The present invention relates to a method for atomizing molten polymeric materials to produce fine polymer particulates and to the polymer particulates, such as polymer spherical powders, fibers and whiskers, formed by such atomization and to atomizing apparatus for polymers, metallic and other materials.
Presently, commercial synthetic organic polymer powders are made by grinding of extruded polymer pellets often under cryogenic temperature conditions. Grinding is undesireable as a result of being highly energy-intensive and sensitive to contamination from the grinding equipment used and from environmental pollution. Due to the erratic nature of the grinding process, it is practically impossible to controlling quality, particle size, and distribution of polymer powders. Ball milling of extruded polymer pellets also suffers from similar problems.
Some polymer materials, such as ultra-low molecular weight polyethylene, cannot even be ground or ball milled to form particulates as a result of their waxy nature.
An object of the present invention is to provide a method for making polymer particulates from polymer materials in a manner that overcomes the aforementioned problems heretofore associated with grinding or ball milling to produce commercial polymer powders.
Another object of the present invention is to provide a method for making polymer particulates from polymer materials that heretofore could not be ground or ball milled to particulate form.
Still another object of the present invention is to provide a method for making polymer particulates by gas atomizing molten polymeric material in a manner to provide controlled particle shape, particle size and particle size distribution.
Still a further object of the present invention is to provide a method for gas atomizing molten polymeric material to form polymer particulates without the need for the addition of flow modifiers, such as oils and greases or molybdenum disulfide, to the polymer material to lower viscosity.
Still an additional object of the present invention is to provide atomized polymer particulates with particle shape, particle size and particle size distribution controlled as desired.
Another object of the present invention is to provide atomized polymer particulates which heretofore could not be produced on a mass-production basis by grinding or ball milling of extruded material.
Still a further object of the present invention is to provide atomized polymer particulates of improved quality with reduced contamination from flow modifiers and other additives heretofore used.
A further object of the invention is to provide improved atomizing apparatus for gas atomizing fluid polymer materials, inorganic materials, metallic materials and others.
The present invention involves a method for making polymer particulates by providing a fluid (e.g. molten) polymer material, which may include virgin polymer material and/or recycled polymer waste material, under conditions to avoid thermal degradation of the polymer material, atomizing the melt using gas jet means in a manner to form atomized droplets, and treating (e.g. cooling) the droplets to form solid polymer particulates, which are collected for further processing. Certain atomization parameters can be controlled for a given polymer material to produce polymer particulates with controlled particle shape, particle size, and particle size distribution. For example, atomization parameters can be controlled to produce polymer spheres, polymer fibers or whiskers, and combinations thereof.
In an illustrative embodiment of the present invention, polymer material is heated in a melting vessel to an atomization temperature under an inert or other non-reactive gaseous atmosphere effective to increase the thermal degradation temperature of the polymer material. The gaseous atmosphere is used to raise the thermal degradation temperature of the polymer melt. The atomization temperature is selected to be between the polymer melting temperature and the raised thermal degradation temperature so as to avoid polymer thermal degradation at the atomization temperature.
A stream of molten polymer is supplied from the melting vessel to an atomizing nozzle typically by providing gas pressure on the polymer melt in the melting vessel to force it through a melt supply or pour tube to the atomizing nozzle. The polymer stream is supplied to the atomizing nozzle where one or more gas jets are directed at the molten polymer stream to disintegrate it to form fine polymer droplets. The atomized polymer droplets fall by gravity and rapidly cool to form solidified polymer particulates that are collected in a collection chamber below the atomizing nozzle.
The present invention also provides atomized polymer particulates that can comprise spherical polymer powders, polymer fibers or whiskers, and combinations thereof. Spherical polymer particulates can be atomized in the size range of about 200 microns or less mean particle size. Spherical particles in the size range of about 5 to about 200 microns diameter are especially useful as a product powder of the invention for a wide variety of commercial applications, such as paint additives, sprayable coating materials and the like. Fiber or whisker shaped polymer particulates having a diameter of 30 microns or less and length of about 1 millimeter to 1 centimeter also can be produced by the invention. Semicrystalline and/or amorphous polymer particulates can be made by the atomization method.
The present invention is advantageous in that polymer particulates can be made from polymers which heretofore could not be ground or ball milled to this end due to their waxy nature. Moreover, polymer particulates can be made with controlled particle shape, particle size and particle size distribution without the need for the addition of flow modifiers, such as oils and greases or molybdenum disulfide, to the polymer material to lower viscosity and with reduced energy consumption compared to grinding and ball milling. As a result, atomized polymer particulates can be made having improved quality with reduced contamination. Polymer particulates can be made from virgin polymer materials and/or recycled polymer waste materials.
The present invention also involves atomizing apparatus comprising means for discharging atomizing gas and a supply tube for molten material to be atomized by the atomizing gas wherein the supply tube includes a discharge end positioned proximate the atomizing gas and having a plurality of channels for improving distribution of the molten material toward the atomizing gas for atomization thereby in a manner to control particle size, particle shape and particle size distribution. The atomizing apparatus can be used to atomize polymer materials, inorganic materials, metals and alloys, and other molten or liquid materials.
The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.